1. Field of the Invention
The present invention relates to a switching control circuit that is integrated circuit-compatible and preferably for use in a switching power supply apparatus, and a switching power supply apparatus including such a switching control circuit.
2. Description of the Related Art
In a switching power supply apparatus, a switching control IC includes circuits used for realizing various kinds of functions such as output control, activation, overcurrent protection, overvoltage protection, standby, and power factor improvement. By these functions being increased, the enhanced functionality of a switching control IC is promoted.
In order for each function of the switching control IC to be set so as to be compatible with the operation specification of an application, a plurality of terminals used for an interface with an external circuit are necessary for each function. Therefore, when the number of incorporated functions increases, the number of terminals increases naturally. When the number of terminals increases, the package of the switching control IC becomes large, which leads to an increase in the cost unit price of an IC.
For example, a switching power supply control IC having a normal-time operation mode and a standby-time operation mode is disclosed in Japanese Unexamined Patent Application Publication No. 2007-14081.
FIG. 1 is the circuit diagram of the switching power supply control IC disclosed in Japanese Unexamined Patent Application Publication No. 2007-14081. A manual switching/automatic switching selection signal input terminal ATSTB, a MODE terminal, a SEL unit (comparator), an AUTOH unit (automatic switching circuit), an AUTOL unit (automatic switching circuit), and various kinds of gate circuits NOT1, NAND1, NAND2, OR2, NOR1, NOR2, an NchMOSFET (MN1), and the like are included.
A manual switching/automatic switching determination circuit is configured by the SEL-unit comparator and a threshold voltage VTH_SEL within the control IC, and a comparator output signal is output to the NAND1 and the OR2. The outputs of the NAND1 and the OR2 are input to the NAND2, and become a signal for driving the gate of the NchMOSFET (MN1). In other words, when an ATSTB terminal voltage is higher than the VTH_SEL, a High or Low signal corresponding to a STB terminal emerges in the output of the NAND2, and on the contrary, when the ATSTB terminal voltage is lower than the VTH_SEL, a signal emerges that is obtained by combining the outputs of an AUTOH into which an FB terminal voltage is input and an AUTOL owing to the NOR1. The AUTOH and AUTOL are automatic switching circuits, and include threshold voltages VTH_H and VTH_L and the gates NOR1 and NOR2.
A circuit including an internal power supply VDD, the MN1, a constant current source connected to the drain terminal of the MN1, and the MODE terminal is an operation-delay circuit, and a delay-time setting capacitor is connected to the MODE terminal. When the output of the NAND2 is High, the MODE terminal is fixed to GND owing to the MN1, and in the normal-time operation mode. By an NAND2 output being inverted and the MN1 being turned off, charge is performed on the capacitor connected to the terminal, using a constant current, and a delay time is generated. When a MODE terminal voltage has increased and become High, an OnTB-unit signal of an OR1 output, which specifies a burst operation, is input to the AND1 and combined with a PWM signal, and the standby-time operation mode where a burst switching operation is performed is set.
In the switching power supply control IC illustrated in FIG. 1, the manual switching/automatic switching selection signal input terminal ATSTB is necessary in order to perform selection of a standby mode, and it is difficult to set the validity/invalidity of the standby mode without an increase in the number of terminals.